Electrode active material for nonaqueous secondary batteries, and nonaqueous secondary battery using same

ABSTRACT

wherein Y1 and Y2 are identical or different and represent an oxygen atom, a sulfur atom, or a selenium atom, R1 to R8 are identical or different and represent an oxygen atom or a group represented by —OLi, R9 to R12 are identical or different and represent a hydrogen atom or an organic group, and bonds that are each represented by a solid line and a dashed line indicate a single bond or a double bond.

TECHNICAL FIELD

The present invention relates to an electrode active material fornon-aqueous secondary batteries and a non-aqueous secondary batterycontaining the electrode active material.

BACKGROUND ART

Non-aqueous secondary batteries, such as lithium-ion secondarybatteries, are used in a variety of electric power sources. Studies havebeen conducted on applications of non-aqueous secondary batteries inelectric vehicles, and there is demand for safer battery materials witha higher energy density. Currently, lithium secondary batteries utilizean inorganic material containing a rare heavy metal, such as lithiumcobalt oxide (LiCoO₂), for the cathode material (cathode activematerial) and a carbon material, such as graphite, for the anodematerial. In particular, such inorganic materials containing a rareheavy metal used in cathodes are resource-constrained, and there arealso concerns about their environmental impact.

Organic materials with redox activity have attracted attention as a raremetal-free, more environment-friendly material. One of such potentialmaterials is 1,4-benzoquinones, which are known to undergotwo-electron-transfer redox reaction:

However, due to their high sublimation tendency, 1,4-benzoquinones arenot easily formed into an electrode.

Cycle characteristics are already known to improve when condensationoccurs at redox sites, and a known example is the charge and dischargebehavior of pentacenetetrone (e.g., PTL 1). A naphthazarin dilithiumsalt that, in view of these findings, has an expanded conjugated systemof a 1,4-benzoquinone, and that is in the form of a lithium salt:

exhibits a low sublimation tendency, and is capable of undergoing a4-electron-transfer redox reaction, with its stoichiometric capacitybeing 530 mAh/g, which is higher than the stoichiometric capacity of1,4-benzoquinones of 496 mAh/g. However, the actually obtainablecapacity is about half the stoichiometric value, and the cyclecharacteristics are also insufficient.

CITATION LIST Patent Literature

PTL 1: JP2012-155884

SUMMARY OF INVENTION Technical Problem

The low charge-and-discharge characteristics of the naphthazarindilithium salt are due to the dissolution of the active material in theelectrolyte during charge and discharge. Additionally, the ringcondensation at redox sites improves the cycle characteristics butlowers the voltage when ring condensation occurs in a conjugated system.

From these viewpoints, an object of the present invention is to providean electrode active material for non-aqueous secondary batteries that isless likely to dissolve in an electrolyte during charge and discharge,and that exhibits an excellent discharge capacity and excellentcharge-and-discharge cycle characteristics.

Solution to Problem

The present inventors conducted extensive research to achieve theobject, and found that a compound formed such that naphthazarinskeletons are condensed with a non-conjugated ring such as a dithiinring solves the problem described above, and provides an electrodeactive material for non-aqueous secondary batteries that is less likelyto dissolve in an electrolyte during charge and discharge, and thatexhibits an excellent discharge capacity and excellentcharge-and-discharge cycle characteristics. After conducting furtherresearch based on these findings, the inventors completed the presentinvention. Specifically, the present invention includes the followingsubject matter.

Item 1.

An electrode active material for non-aqueous secondary batteries, theelectrode active material comprising a compound represented by formula(1):

wherein Y¹ and Y² are identical or different and represent an oxygenatom, a sulfur atom, or a selenium atom, R¹to R⁸ are identical ordifferent and represent an oxygen atom or a group represented by OLi, R⁹to R¹² are identical or different and represent a hydrogen atom or anorganic group, and bonds that are each represented by a solid line and adashed line indicate a single bond or a double bond.

Item 2.

The electrode active material for non-aqueous secondary batteriesaccording to item 1, wherein Y¹ and Y² in formula (1) are both a sulfuratom.

Item 3.

The electrode active material for non-aqueous secondary batteriesaccording to item 1 or 2, wherein R¹to R⁴ are identical, and R⁵ to Wareidentical in formula (1).

Item 4.

The electrode active material for non-aqueous secondary batteriesaccording to any one of items 1 to 3, wherein R⁹ to R¹² are identical ordifferent and represent a hydrogen atom, an alkyl group, an alkoxygroup, an aryl group, or a carboxy group in formula (1).

Item 5.

The electrode active material for non-aqueous secondary batteriesaccording to any one of items 1 to 4, wherein the electrode activematerial is a cathode active material for non-aqueous secondarybatteries, the cathode active material comprising a compound representedby formula (1A):

wherein Y¹, Y², R¹ to R⁴, and R⁹ to R¹² are as defined above, and bondsthat are each represented by a solid line and a dashed line indicate asingle bond or a double bond.

Item 6.

The electrode active material for non-aqueous secondary batteriesaccording to any one of items 1 to 4, wherein the electrode activematerial is an anode active material for non-aqueous secondarybatteries, the anode active material comprising a compound representedby formula (1B):

wherein Y¹, Y², and R⁵ to R¹² are as defined above, and bonds that areeach represented by a solid line and a dashed line indicate a singlebond or a double bond.

Item 7.

An electrode for non-aqueous secondary batteries, the electrodecomprising the electrode active material for non-aqueous secondarybatteries of any one of items 1 to 6.

Item 8.

A non-aqueous secondary battery comprising the electrode for non-aqueoussecondary batteries of item 7.

Item 9.

The non-aqueous secondary battery according to item 8, wherein thenon-aqueous secondary battery is a rocking-chair battery.

Advantageous Effects of Invention

Due to its wide molecular plane and strong π-π interaction betweenmolecules, the electrode active material for non-aqueous secondarybatteries according to the present invention is resistant to dissolutionin an electrolyte caused by charge and discharge, and exhibits improvedcycle characteristics.

The electrode active material for non-aqueous secondary batteriesaccording to the present invention, due to the presence of an oxygenatom, a sulfur atom, or a selenium atom of the sp³ structure, has itsconjugated system cut off. Thus, decreases in voltage caused by ringcondensation is unlikely to occur. The electrode active material fornon-aqueous secondary batteries according to the present invention alsohas 8 Li ions inserted per molecule during discharge, and the valence ofthe molecule accordingly varies between −8 to 0. Thus, the electrodeactive material for non-aqueous secondary batteries according to thepresent invention exhibits a discharge capacity of 400 mAh/g or more,which is attributed to the reaction involving 8 electrons per molecule.This value is nearly three times the discharge capacity of 140 mAh/g oflithium cobalt oxide, which is an existing cathode material for lithiumsecondary batteries.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating the results of Test Example 1 (acharge-and-discharge test on a half-cell containing the compound ofExample 1 as a cathode active material: initial discharge curve).

FIG. 2 is a graph illustrating the results of Test Example 1 (acharge-and-discharge test on half-cells containing the compound ofExample 1 or Comparative Example 1 as a cathode active material: cyclecharacteristics).

FIG. 3 is a graph illustrating the results of Test Example 2 (acharge-and-discharge test on an full-cell containing the compound ofExample 1 as a cathode active material and as an anode active material:initial charge and discharge curve).

FIG. 4 is a graph illustrating the results of Test Example 2 (acharge-and-discharge test on an full-cell containing the compound ofExample 1 as a cathode active material and as an anode active material:cycle characteristics).

DESCRIPTION OF EMBODIMENTS 1. Electrode Active Material for Non-AqueousSecondary Batteries

The electrode active material for non-aqueous secondary batteriesaccording to the present invention comprises a compound represented byformula (1):

wherein Y¹ and Y² are identical or different and represent an oxygenatom, a sulfur atom, or a selenium atom, R¹to R⁸ are identical ordifferent and represent an oxygen atom or a group represented by OLi, R⁹to R¹² are identical or different and represent a hydrogen atom or anorganic group, and bonds that are each represented by a solid line and adashed line indicate a single bond or a double bond.

In formula (1), Y¹ and Y²each represent an oxygen atom, a sulfur atom,or a selenium atom. The electrode active material for non-aqueoussecondary batteries according to the present invention, due to thepresence of an oxygen atom, a sulfur atom, or a selenium atom of sp³structure, has its conjugated system cut off; and decreases in voltagecaused by ring condensation is thus unlikely to occur. From thestandpoint of greater prevention of decreases in voltage and greaterimprovement in capacity (in particular, discharge capacity), a sulfuratom is preferable. Although Y¹ and Y² may be identical or different, Y¹and Y² are preferably identical from the standpoint of simplicity ofsynthesis.

The organic group represented by R⁹ to R¹² in formula (1) includesalkyl, alkoxy, aryl, and carboxy.

The alkyl as the organic group represented by R⁹ to R¹² in formula (1)is preferably alkyl having 1 to 6 carbon atoms (in particular 1 to 4),such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl,sec-butyl, and tert-butyl. The alkyl for use may be either linear alkylor branched alkyl. This alkyl may also be substituted. The substituentsare not particularly limited, and include hydroxy and halogen (e.g., afluorine atom, a chlorine atom, a bromine atom, and an iodine atom). Forsuch substituted alkyl, the number of substituents is not particularlylimited, and is, for example, 1 to 3.

The alkoxy as the organic group represented by R⁹ to R¹² in formula (1)is preferably alkoxy having 1 to 6 carbon atoms (in particular 1 to 4),such as methoxy, ethoxy, n-propoxy, isopropoxy, n-butyloxy, isobutyloxy,sec-butyloxy, and tert-butyloxy. The alkoxy for use may be either linearalkoxy or branched alkoxy. This alkoxy may also be substituted. Thesubstituents are not particularly limited, and include hydroxy andhalogen (e.g., a fluorine atom, a chlorine atom, a bromine atom, and aniodine atom). For such substituted alkoxy, the number of substituents isnot particularly limited, and is, for example, 1 to 3.

Examples of the aryl as the organic group represented by R⁹ to R¹² informula (1) include phenyl, naphthyl, anthranil, phenanthryl, biphenyl,and pyridyl. This aryl may also be substituted. The substituents are notparticularly limited, and include hydroxy and halogen (e.g., a fluorineatom, a chlorine atom, a bromine atom, and an iodine atom). For suchsubstituted aryl, the number of substituents is not particularlylimited, and is, for example, 1 to 3.

R⁹ to R¹² in formula (1) are preferably a hydrogen atom, from thestandpoint of simplicity of synthesis, capacity, andcharge-and-discharge cycle characteristics.

R¹ to R⁸ in formula (1) each represent an oxygen atom or a grouprepresented by —OLi. Although R¹ to R⁸ may be identical or different, itis preferred that R¹ to R⁴ are identical, while R⁵ to R⁸ are identical,from the standpoint of simplicity of synthesis, capacity, andcharge-and-discharge cycle characteristics.

In the full charge mode, R¹ to R⁸ in the electrode active material fornon-aqueous secondary batteries according to the present invention areall an oxygen atom, and a compound represented by formula (1-1) islikely to form:

wherein Y¹, Y², and R⁹ to R¹² are as defined above.

As discharge proceeds, a lithium atom is inserted into some of theoxygen atoms to form a compound represented by formula (1-2):

wherein Y¹, Y², and R⁹to R¹² are as defined above. Further, when thedischarge is performed completely thereafter, R¹ to R⁸ all become agroup represented by —OLi, and a compound represented by formula (1-3)is likely to form:

wherein Y¹, Y², and R⁹to R¹² are as defined above.

Of these compounds represented by formulas (1-1) to (1-3), the compoundsrepresented by formula (1-1) and (1-2) can have a lithium atom insertedinto an oxygen atom to have a group represented by —OLi. Thus, thecompounds represented by formulas (1-1) and (1-2) are usable in areaction of inserting a lithium atom (i.e., usable as a cathode activematerial for non-aqueous secondary batteries). Accordingly, a compoundrepresented by formula (1A) is usable as a cathode active material fornon-aqueous secondary batteries:

wherein Y¹, Y², R¹ to R⁴, and R⁹ to R¹²are as defined above, and bondsthat are each represented by a solid line and a dashed line indicate asingle bond or a double bond.

Of these compounds represented by formulas (1-1) to (1-3), the compoundsrepresented by formulas (1-2) and (1-3) can release a lithium atom fromthe group represented by —OLi to leave an oxygen atom. Thus, thecompounds represented by formulas (1-2) and (1-3) are usable in areaction of releasing a lithium atom (i.e., usable as an anode activematerial for non-aqueous secondary batteries). Accordingly, a compoundrepresented by formula (1B) is usable as an anode active material fornon-aqueous secondary batteries:

wherein Y¹, Y², and R⁵ to R¹² are as defined above, and bonds that areeach represented by a solid line and a dashed line indicate a singlebond or a double bond.

As described above, the compound represented by formula (1-2) is usableboth as a cathode active material and as an anode active material fornon-aqueous secondary batteries. Specifically, the use of the compoundrepresented by formula (1-2) as a cathode active material and an anodeactive material enables the production of a non-aqueous secondarybattery in which the same material is used in the active material ofboth electrodes.

Examples of electrode active materials for non-aqueous secondarybatteries according to the present invention that satisfy the conditionsdescribed above include the following:

Such an electrode active material for non-aqueous secondary batteriesaccording to the present invention that satisfies the conditionsdescribed above is a compound that is less likely to dissolve in anelectrolyte during charge and discharge, and that exhibits excellentcharge-and-discharge cycle characteristics. Due to its wide molecularplane and strong 7E-7E interaction between molecules, the electrodeactive material for non-aqueous secondary batteries according to thepresent invention is resistant to dissolution in an organic electrolytecaused by charge and discharge, and exhibits improvedcharge-and-discharge cycle characteristics.

The electrode active material for non-aqueous secondary batteriesaccording to the present invention is a known compound, or can besynthesized through a known reaction.

For example, the compound represented by formula (1-2) can besynthesized through the following reaction scheme 1.

wherein Y¹, Y², and R⁹ to R¹² are as defined above; and X¹ and X² areidentical or different, and each represents a halogen atom.

The halogen atom represented by X¹ or X²in reaction scheme 1 includes afluorine atom, a chlorine atom, a bromine atom, and an iodine atom.

First, a diacetoxy dihalonaphthoquinone compound (3) is prepared from adihalo dihydroxy naphthoquinone compound (2) (starting material) by aknown method (e.g., by dissolving compound (2) in acetic acid).Thereafter, the diacetoxy dihalonaphthoquinone compound (3) and rubeanicacid are reacted in a solvent (e.g., dimethylformamide) in the presenceof a base (e.g., an amine compound, such as triethylamine), therebypreparing a tetraacetoxy dibenzo thianthrene tetrone (4). Thetetraacetoxy dibenzo thianthrene tetrone is further reacted with alithium compound (e.g., lithium hydroxide), thereby preparing thecompound represented by formula (1-2). The reaction conditions are notparticularly limited, and may be those typically used. When, instead ofa sulfur atom, an oxygen atom or a selenium atom is used for Y¹ and Y²,the use of a suitable oxygen compound or selenium compound, instead ofrubeanic acid, provides the compound represented by formula (1-2). Anelectrode active material for non-aqueous secondary batteries accordingto the present invention other than the compound represented by formula(1-2) can also be synthesized through a known reaction in the samemanner.

2. Non-Aqueous Secondary Battery

A non-aqueous secondary battery containing the electrode active materialfor non-aqueous secondary batteries according to the present inventioncan be produced by a known technique. Such a non-aqueous secondarybattery can have the formation and structure used in traditionally knownnon-aqueous secondary batteries, except that the electrode activematerial for non-aqueous secondary batteries according to the presentinvention is used as a cathode active material and/or as an anode activematerial. Typically, the non-aqueous secondary battery according to thepresent invention contains a cathode, an anode, a non-aqueouselectrolyte, and a separator.

The present invention also allows, as described later, the compoundrepresented by formula (1A), which is an electrode active material fornon-aqueous secondary batteries according to the present invention, tobe used as a cathode active material and the compound represented byformula (1B), which is an electrode active material for non-aqueoussecondary batteries according to the present inventions, to be used asan anode active material.

For example, when the compound represented by formula (1-2) is used inboth the cathode active material and the anode active material, lithiumis released in the cathode during charge to thereby form the compoundrepresented by formula (1-1), while lithium is inserted in the anode toform the compound represented by formula (1-3). During discharge, thereverse reaction occurs in each electrode.

More specifically, the use of the electrode active material fornon-aqueous secondary batteries according to the present inventioninvolves an insertion-extraction reaction of lithium during charge anddischarge. When the electrode active material for non-aqueous secondarybatteries according to the present invention is used in both the cathodeactive material and the anode active material, ions that go in and outof both the cathode and the anode are lithium ions. Thus, it is possibleto form a rocking chair non-aqueous secondary battery (in particular, arocking chair lithium-ion secondary battery) that does not undergochanges in the concentration of the electrolyte during charge anddischarge.

The concept of the “lithium-ion secondary battery,” which is anembodiment of non-aqueous secondary batteries, in the present invention,includes “lithium secondary batteries” containing metallic lithium inthe anode material Additionally, the concept of the “lithium-ionsecondary battery,” which is an embodiment of non-aqueous secondarybatteries, in the present invention, also includes “non-aqueouslithium-ion secondary batteries” containing a non-aqueous electrolyteand “all-solid-state lithium-ion secondary batteries” containing a solidelectrolyte.

(2-1) Cathode

The cathode may be configured such that a mixed cathode layer containinga cathode active material, a binder, etc., is formed on one surface orboth surfaces of a cathode current collector.

This mixed cathode layer is prepared by adding the binder to the cathodeactive material and an optionally added conductive material, blendingthis mixture to form a sheet, and pressing this sheet onto the cathodecurrent collector formed from a metal foil or other materials The mixedcathode layer may also be prepared by adding the binder to the cathodeactive material and an optionally added conductive material, dispersingthe mixture in an organic solvent to prepare a paste for forming a mixedcathode layer (in this case, the binder may be dissolved or dispersed inan organic solvent beforehand), applying the paste onto the surface (onesurface or both surfaces) of the cathode current collector formed from ametal foil or other materials, drying the applied paste to form a mixedcathode layer, and optionally subjecting the layer to a processing step.

When the electrode active material for non-aqueous secondary batteriesaccording to the present invention is used in the cathode activematerial, the electrode active material for use may be the compoundrepresented by formula (1A). When the electrode active material fornon-aqueous secondary batteries according to the present invention isnot used in the cathode active material, the cathode active material foruse is not particularly limited; instead, a material that allows chargeand discharge to proceed at high potential can be used. Examples includelamellar oxides, such as LiMnO₂, LiNiO₂, LiCoO₂, Li(Mn_(x)Ni_(1-x))O₂,Li(Mn_(x)Co_(1-x))O₂, Li(Ni_(y)Co_(1-y))O₂, andLi(Mn_(x)Ni_(y)Co_(1-x-y))O₂; solid solutions, such as Li₂MnO₃—LiNiO₂,Li₂MnO₃—LiCoO₂, and Li₂MnO₃—Li(Ni_(y)Co_(1-y))O₂; silicates, such asLi₂MnSiO₄, Li₂NiSiO₄, Li₂CoSiO₄, Li₂(Mn_(x)Ni_(1-x))SiO₄,Li₂(Mn_(x)Co_(1-x))SiO₄, Li₂(NiCo_(1-y))SiO₄, andLi₂(Mn_(x)NiCo_(1-x-y))SiO₄; borates, such as LiMnBO₃, LiNiBO₃, LiCoBO₃,Li(Mn_(x)Ni_(1-x))BO₃, Li(Mn_(x)Co_(1-x))BO₃, Li(Ni_(y)Co_(1-y))BO₃, andLi(Mn_(x)Ni_(y)Co_(1-x-y))BO₃; V₂O₅; LiV₃O₆; and MnO. In these formulas,0<x<1, 0<y<1, and 0<x+y<1. These cathode active materials can be usedsingly, or in a combination of two or more.

A conductive material for use may be, as with typical non-aqueoussecondary batteries, graphite; carbon black (e.g., acetylene black andKetjenblack); amorphous carbon materials, such as carbon materialsprepared by forming amorphous carbon on the surface; fibrous carbon(e.g., vapor-grown carbon fiber and carbon fiber prepared bycarbonization after spinning the pitch); and carbon nanotubes (a rangeof multi-walled or single-walled carbon nanotubes). For the conductivematerial for the cathode, these listed materials may be used singly, orin a combination of two or more.

Examples of binders include polyvinylidene fluoride (PVDF),polytetrafluoroethylene, polyacrylic acid, and styrene butadiene rubber.

The organic solvent for use in preparing a mixed cathode is notparticularly limited, and examples include N-methylpyrrolidone (NMP). Apaste can be formed from the organic solvent, a cathode active material,a binder, etc.

Regarding the formulation of the mixed cathode layer, it is preferredthat, for example, the cathode active material be present in an amountof 40 to 80 mass %, and that the binder be present in an amount of 20 to60 mass %. When a conductive material is used, it is preferred that thecathode active material be present in an amount of 20 to 60 mass %, thatthe binder be present in an amount of 5 to 20 mass %, and that theconductive material be present in an amount of 30 to 70 mass %.Additionally, the thickness of the mixed cathode layer is preferably 1to 200 pm, per one surface of the current collector.

Examples of cathode current collectors for use include aluminum,stainless steel, nickel, and titanium; and foil, mesh, perforated metal,expanded metal, etc., that are formed from alloys of aluminum, stainlesssteel, nickel, or titanium. Typically, a stainless steel mesh that has athickness of 10 to 200 pm is suitably used.

(2-2) Anode

The anode may be configured such that a mixed anode layer containing ananode active material, a binder, etc., is formed on one surface or bothsurfaces of a anode current collector. A non-aqueous secondary batteryaccording to the present invention in the form of a metallic lithiumsecondary battery can use the metallic lithium as its anode.

This mixed anode layer can be prepared by adding the binder to the anodeactive material and an optionally added conductive material, blendingthe mixture to form a sheet, and pressing the sheet onto the anodecurrent collector formed from a metal foil or other materials. The mixedanode layer may also be prepared by adding the binder to the anodeactive material and an optionally added conductive material, dispersingthe mixture in an organic solvent to prepare a paste for forming a mixedanode layer (in this case, the binder may be dissolved or dispersed inan organic solvent beforehand), applying the paste onto the surface (onesurface or both surfaces) of the anode current collector formed from ametal foil or other materials, drying the applied paste to form a mixedanode layer, and optionally subjecting the layer to a processing step.

When the electrode active material for non-aqueous secondary batteriesaccording to the present invention is used in the anode active material,the electrode active material for use may be the compound represented byformula (1B). When the electrode active material for non-aqueoussecondary batteries according to the present invention is not used inthe anode active material, the anode active material for use is notparticularly limited; examples of anode active materials for use includegraphite (e.g., natural graphite and artificial graphite),sintering-resistant carbon, lithium metal, tin, silicon, alloyscontaining these materials, and SiO. Lithium metal, lithium alloy, orthe like are preferably used in the active material for metallic lithiumprimary batteries and metallic lithium secondary batteries, whilematerials capable of doping and undoping lithium ions (e.g., graphite,including natural graphite and artificial graphite, andsintering-resistant carbon) are preferably used in the active materialfor lithium-ion secondary batteries. These anode active materials may beused singly, or in a combination of two or more.

A conductive material for use may be, as with typical non-aqueoussecondary batteries, graphite; carbon black (e.g., acetylene black andKetjenblack); amorphous carbon materials, such as carbon materialsprepared by forming amorphous carbon on the surface; fibrous carbon(e.g., vapor-grown carbon fiber and carbon fiber prepared bycarbonization after spinning the pitch); and carbon nanotubes (a rangeof multi-walled or single-walled carbon nanotubes). For the conductivematerial for the cathode, these listed substances may be used singly orin a combination of two or more.

Examples of binders include polyvinylidene fluoride (PVDF),polytetrafluoroethylene, polyacrylic acid, and styrene butadiene rubber.

The organic solvent for use in preparing a mixed anode is notparticularly limited, and examples include N-methylpyrrolidone (NMP). Apaste can be formed from the organic solvent, an anode active material,a binder etc.

Regarding the formulation of the mixed anode layer, it is preferredthat, for example, the anode active material be present in an amount of40 to 80 mass %, and that the binder be present in an amount of 20 to 60mass %. When a conductive material is used, it is preferred that theanode active material be present in an amount of 20 to 60 mass %, thatthe binder be present in an amount of 5 to 20 mass %, and that theconductive material be present in an amount of 30 to 70 mass %.Additionally, the thickness of the mixed anode layer is preferably 1 to200 per one surface of the current collector.

Examples of anode current collectors for use include aluminum, stainlesssteel, nickel, and titanium; and foil, mesh, perforated metal, expandedmetal, etc., that are formed from alloys of aluminum, stainless steel,nickel, or titanium. Typically, a stainless steel mesh that has athickness of 10 to 200 μm is suitably used.

(2-3) Non-Aqueous Electrolyte

A non-aqueous electrolyte for use may be a solution (organicelectrolyte) prepared by dissolving a lithium salt, such as lithiumperchlorate or LiPF₆, in a solvent formed of at least one knownsubstance of, for example, ethylene carbonate, dimethyl carbonate, anddiethyl carbonate. A solid electrolyte for use may also be an inorganicsolid electrolyte (e.g., Li₂S-P₂S₅-based electrolyte andLi₂S-GeS₂-P₂S₅-based electrolyte).

(2-4) Separator

The cathode and the anode described above are used in the form of, forexample, a laminated electrode prepared by laminating the cathode andthe anode with an interjacent separator between them, or in the form ofa spiral-wound electrode prepared by further winding the laminatedelectrode into a spiral shape.

The separator preferably has sufficient strength and can retain as muchelectrolyte as possible. From these viewpoints, the separator ispreferably a microporous film, a non-woven fabric, a filter, etc., thathas a thickness of 10 to 50 μm and a porosity of 30 to 70%, and thatcontains at least one of polyethylene, polypropylene, anethylene-propylene copolymer, glass, and the like.

EXAMPLES

The following describes the present invention in detail with referenceto Examples. However, the present invention is not limited to theExamples.

Example 1

1,4,8,11-Tetrahydroxy dibenzo[b,i]thianthrene-5,7,12,14-tetronetetralithium salt (1) was synthesized through the following route.First, 0.93 g of 2,3-dichloro-5,8-dihydroxy-1,4-naphthoquinone wasdissolved in 24 mL of acetic anhydride, and refluxed for 8 hours. Aftercooling, the precipitate was filtered, thereby obtaining 0.92 g of5,8-diacetoxy-2,3-dichloro-1,4-naphthoquinone. 0.85 g of the synthesized5,8-diacetoxy-2,3-dichloro-1,4-naphthoquinone and 0.17 g of rubeanicacid were dissolved in 13 mL of dimethylformamide, and then 1 mL oftriethylamine was added thereto, followed by stirring at 50° C. for 10hours. After cooling, the precipitate was filtered, and washed withwater, thereby obtaining 0.72 g of 1,4,8,11-tetraacetoxydibenzo[b,i]thianthrene-5,7,12,14-tetrone. Thereafter, 0.64 g of theobtained solid was dissolved in a lithium hydroxide solution, andstirred at room temperature for 2 hours. After evaporation of thesolvent, the residue was washed with ethanol, thereby obtaining 0.39 gof 1,4,8,11-tetrahydroxy dibenzo [b,i]thianthrene-5,7,12,14-tetronetetralithium salt: melting point:>400° C., ¹H-NMR (400 MHz, DMSO-₆₎:δ6.54 (s, 4 H).

Comparative Example 1

A known cathode active material (a naphthazarin dilithium salt) was usedas a cathode active material for Comparative Example 1.

Test Example 1 Evaluation of Electrode (Half-cell)

The compound synthesized in Example 1 (cathode active material) wasmixed with acetylene black (conductive material) and FIFE (binder) in aratio of 4:5:1 (active material:conductive material:binder on a massbasis) to prepare a sheet with a thickness of 90 μm. This sheet waspressed onto a stainless steel mesh (thickness: 100 μm), therebypreparing a cathode. Then, a coin-shaped cell for testing was preparedusing this cathode for the cathode material, a lithium foil for theanode material, lithiumbis(trifluoromethanesulfonyl)imide/tetrahydropyran (3 mol/L) for theelectrolyte, and a glass filter for the separator. Acharge-and-discharge test was performed on this cell in the atmosphereat 30° C. at a current density of 20 mA/g in the potential range of1.5-4.2 V (vs. Li⁺/Li).

FIG. 1 illustrates the initial discharge curve. The discharge curve hasa two-stage flat potential part around 3.5 V (vs. Li⁺/Li), and aone-stage flat potential part around 2 V (vs. Li⁺/Li), indicating amulti-electron reaction. The initial discharge capacity was as high as413 mAh/g, which is close to 462 mAh/g, the stoichiometric value in thereaction in which 8 electrons are assumed to migrate per molecule. Theelectrode had a discharge capacity almost three times the dischargecapacity of lithium cobalt oxide (140 mAh/g), which is a cathodematerial of a typical lithium-ion secondary battery.

FIG. 2 illustrates a comparison between the cycle change in thedischarge capacity of an electrode prepared using compound (1) ofExample 1 for the cathode active material and the cycle characteristicsof an electrode prepared using a naphthazarin dilithium salt (thecompound of Comparative Example 1) for the cathode active material. Thecell containing the compound of Example 1 for the cathode activematerial exhibited a smaller decrease in capacity even when charge anddischarge were repeated, and maintained a capacity of about 353 mAh/gafter 20 cycles, exhibiting excellent cycle characteristics. Incontrast, the cell containing a naphthazarin dilithium salt for thecathode active material exhibited an initial capacity of 259 mAh/g,which was about half of the stoichiometric value (530 mAh/g), and alsoexhibited a gradually decreasing discharge capacity as the number ofcycles increased, with a capacity of 67 mAh/g after 20 cycles. Itappears that because the compound of Example 1 has a wider molecularplane and stronger π-π interaction between molecules than thenaphthazarin dilithium salt, the dissolution of the compound of Example1 in the organic electrolyte caused by charge and discharge was reduced,and this led to improved cycle characteristics.

Test Example 2 Evaluation of Electrode (Rocking Chair Full-Cell)

Two electrodes were prepared using compound (1) of Example 1 for theactive material as described above; and an full-cell was prepared usingthe electrodes, one in the cathode and the other in the anode. Lithiumbis(trifluoromethanesulfonyl)imide/sulfolane (1 mol/L) was used for theelectrolyte, and a glass filter was used for the separator. Acharge-and-discharge test was performed on the prepared full-cell in theatmosphere at 30° C. at a current density of 20 mA/g, in the voltagerange of 0.0-2.5 V (difference of electrical potential).

FIG. 3 illustrates charge and discharge curves. The discharge curve alsohas a two-stage flat potential part, indicating a multi-electronreaction. The discharge capacity determined by FIG. 3 was 173 mAh/g,which is close to a stoichiometric value of 231 mAh/g in the reaction inwhich 4 electrons are assumed to migrate per molecule. The resultsreveal that compound (1) of Example 1 has the function of both thecathode active material and the anode active material. As illustrated inFIG. 4, the cycle characteristics were also relatively preferable.

1. An electrode active material for non-aqueous secondary batteries, theelectrode active material comprising a compound represented by formula(1):

wherein Y¹ and Y² are identical or different and represent an oxygenatom, a sulfur atom, or a selenium atom, R¹ to R⁸ are identical ordifferent and represent an oxygen atom or a group represented by —OLi,R⁹ to R¹² are identical or different and represent a hydrogen atom or anorganic group, and bonds that are each represented by a solid line and adashed line indicate a single bond or a double bond.
 2. The electrodeactive material for non-aqueous secondary batteries according to claim1, wherein Y¹ and Y² in formula (1) are both a sulfur atom.
 3. Theelectrode active material for non-aqueous secondary batteries accordingto claim 1, wherein R¹ to R⁴ are identical, and R⁵ to R⁸ are identicalin formula (1).
 4. The electrode active material for non-aqueoussecondary batteries according to claim 1, wherein R⁹ to R¹² areidentical or different and represent a hydrogen atom, an alkyl group, analkoxy group, an aryl group, or a carboxy group in formula (1).
 5. Theelectrode active material for non-aqueous secondary batteries accordingto claim 1, wherein the electrode active material is a cathode activematerial for non-aqueous secondary batteries, the cathode activematerial comprising a compound represented by formula (1A):

wherein Y¹, Y², R¹ to R⁴, and R⁹ to R¹² are as defined above, and bondsthat are each represented by a solid line and a dashed line indicate asingle bond or a double bond.
 6. The electrode active material fornon-aqueous secondary batteries according to claim 1, wherein theelectrode active material is an anode active material for non-aqueoussecondary batteries, the anode active material comprising a compoundrepresented by formula (1B):

wherein Y¹, Y², and R⁵ to R¹² are as defined above, and bonds that areeach represented by a solid line and a dashed line indicate a singlebond or a double bond.
 7. An electrode for non-aqueous secondarybatteries, the electrode comprising the electrode active material fornon-aqueous secondary batteries of claim
 1. 8. A non-aqueous secondarybattery comprising the electrode for non-aqueous secondary batteries ofclaim
 7. 9. The non-aqueous secondary battery according to claim 8,wherein the non-aqueous secondary battery is a rocking-chair battery.10. The electrode active material for non-aqueous secondary batteriesaccording to claim 2, wherein R¹ to R⁴ are identical, and R⁵ to R⁸ areidentical in formula (1).
 11. The electrode active material fornon-aqueous secondary batteries according to claim 2, wherein R⁹ to R¹²are identical or different and represent a hydrogen atom, an alkylgroup, an alkoxy group, an aryl group, or a carboxy group in formula(1).
 12. The electrode active material for non-aqueous secondarybatteries according to claim 3, wherein R⁹ to R¹² are identical ordifferent and represent a hydrogen atom, an alkyl group, an alkoxygroup, an aryl group, or a carboxy group in formula (1).
 13. Theelectrode active material for non-aqueous secondary batteries accordingto claim 2, wherein the electrode active material is a cathode activematerial for non-aqueous secondary batteries, the cathode activematerial comprising a compound represented by formula (1A):

wherein Y¹, Y², R¹ to R⁴, and R⁹ to R¹² are as defined above, and bondsthat are each represented by a solid line and a dashed line indicate asingle bond or a double bond.
 14. The electrode active material fornon-aqueous secondary batteries according to claim 3, wherein theelectrode active material is a cathode active material for non-aqueoussecondary batteries, the cathode active material comprising a compoundrepresented by formula (1A):

wherein Y¹, Y², R¹ to R⁴, and R⁹ to R¹² are as defined above, and bondsthat are each represented by a solid line and a dashed line indicate asingle bond or a double bond.
 15. The electrode active material fornon-aqueous secondary batteries according to claim 4, wherein theelectrode active material is a cathode active material for non-aqueoussecondary batteries, the cathode active material comprising a compoundrepresented by formula (1A):

wherein Y¹, Y², R¹ to R⁴, and R⁹ to R¹² are as defined above, and bondsthat are each represented by a solid line and a dashed line indicate asingle bond or a double bond.
 16. The electrode active material fornon-aqueous secondary batteries according to claim 2, wherein theelectrode active material is an anode active material for non-aqueoussecondary batteries, the anode active material comprising a compoundrepresented by formula (1B):

wherein Y¹, Y², and R⁵ to R¹² are as defined above, and bonds that areeach represented by a solid line and a dashed line indicate a singlebond or a double bond.
 17. The electrode active material for non-aqueoussecondary batteries according to claim 3, wherein the electrode activematerial is an anode active material for non-aqueous secondarybatteries, the anode active material comprising a compound representedby formula (1B):

wherein Y¹, Y², and R⁵ to R¹² are as defined above, and bonds that areeach represented by a solid line and a dashed line indicate a singlebond or a double bond.
 18. The electrode active material for non-aqueoussecondary batteries according to claim 4, wherein the electrode activematerial is an anode active material for non-aqueous secondarybatteries, the anode active material comprising a compound representedby formula (1B):

wherein Y¹, Y², and R⁵ to R¹² are as defined above, and bonds that areeach represented by a solid line and a dashed line indicate a singlebond or a double bond.
 19. An electrode for non-aqueous secondarybatteries, the electrode comprising the electrode active material fornon-aqueous secondary batteries of claim
 2. 20. An electrode fornon-aqueous secondary batteries, the electrode comprising the electrodeactive material for non-aqueous secondary batteries of claim 3.